The subject invention relates, generally, to alloys and, more particularly, to ternary and quaternary homogeneous alloys and to methods for making such alloys.
III-V and II-VI compound semiconductors with variable band gaps (Eg) and lattice constants are needed for numerous electronic and optoelectronic applications, including: light emitting diodes, laser diodes, photodetectors, solar and photovoltaic cells, high speed switches, and the like. Directional solidification from the melt is by far the fastest, cheapest, most reliable, and, therefore, the preferred method for producing large scale, device grade, single crystal substrates. Unfortunately, only binary compounds (like GaAs, GaSb, and InP) have been successfully produced in large quantities from melts having discrete energy band gaps and lattice constants. In principle, the band gap and the lattice constant can be tuned in ternary, quaternary, or higher order systems by adjusting the composition of the substitutional cations and anions. However, in practice melt-grown ternary and higher order compounds are compositionally inhomogeneous (see, e.g., Bachmann et al., xe2x80x9cMelt and Solution Growth of Bulk Single Crystals of Quaternary III-V Alloysxe2x80x9d, Progress in Crystal Growth and Characterization, 2(3):171-206 (1979) (xe2x80x9cBachmannxe2x80x9d)) and exhibit high density of defects, such as cracks, inclusions, precipitates, dendrites, and dislocations. These defects are due to several reasons, including: large lattice mismatch between the constituent binaries, wide separation between the liquidus and solidus curves in the pseudo-binary phase diagrams, differences in thermal expansion coefficients of the binary compounds, and miscibility gaps. Attempts to grow uniform crystals via external solute feeding of the depleted components, for example, by the method described U.S. Pat. No. 5,047,112 to Ciszek, have not been successful, especially for concentrated alloy compositions, primarily due to large temperature differences between the solidus and liquidus curves. Likewise, attempts to employ methods used to produce uniformly doped binary materials, such as those described in Ostrogorsky, xe2x80x9cNumerical Simulation of Single Crystal Grown by Submerged Heater Method,xe2x80x9d J. Crystal Growth, 104:233-238 (1990) and U.S. Pat. No. 5,047,113 to Ostrogorsky to grow ternary or quaternary alloys have proved unsuccessful. Moreover, post growth treatments, like zone leveling or prolonged annealing of the solidified ingot, have not been effective in eliminating these extended defects see, e.g., Bachmann). Enhanced mixing in the melt near the solid-liquid interface during crystal growth helps in reducing cracks, but the axial segregation still persists (see, Dutta et al., xe2x80x9cSuppression of Cracks in InxGa1xe2x88x92xSb Crystals through Forced Convection in the Meltxe2x80x9d, Journal of Crystal Growth, 194:1-7 (1998) (xe2x80x9cDuttaxe2x80x9d).
Ternary and quaternary semiconductor materials are currently produced in the form of thin layers by non-equilibrium growth techniques (from diluted solutions and vapor phase) on binary substrates using buffer layers to relieve misfit related stresses at the epilayer-substrate interface. One disadvantage of epitaxial technology is its high cost. In addition, the buffer layer technology is not optimized for all systems, and, often devices exhibit large leakage currents due to poor interfacial regions.
From extensive experimental work, it has become clear that a high quality substrate is needed to obtain high-performance reliable devices. Substrates with tunable band gap and lattice constants would open up numerous possibilities of interesting band gap engineering in homo- and hetero-epitaxial devices and would significantly simplify the fabrication cycle. Hence, the overall cost of the final device would be reduced significantly. In addition, substrates with tunable band gap and lattice constants would facilitate the production of device structures and designs that have not been possible heretofore. The present invention is directed to meeting this need.
The present invention relates to a method for preparing a homogeneous ternary or quaternary alloy from a quaternary melt. The method includes providing a family of phase diagrams for the quaternary melt which shows (i) composition/temperature data, (ii) tie lines connecting equilibrium liquid and solid compositions, and (iii) isotherms representing boundaries of a miscibility gap. Based on the family of phase diagrams, a quaternary melt composition and an alloy growth temperature is selected. A quaternary melt having the selected quaternary melt composition is provided, and a ternary or quaternary alloy is grown from the quaternary melt at the selected alloy growth temperature.
The present invention also relates to another method for preparing a homogeneous ternary or quaternary alloy from a ternary or quaternary melt. The method includes providing ternary or quaternary melt having a ternary or quaternary melt composition which includes at least three or four melt elements. The ternary or quaternary melt is in solution equilibrium with a solid binary material. The solid binary material has a melting temperature greater than that of the ternary or quaternary alloy""s solidus temperature, and the solid binary material includes two of the at least three or four melt elements. The method further includes heating the ternary or quaternary melt to the ternary or quaternary alloy""s solidus temperature, and agitating the ternary or quaternary melt mechanically under conditions effective to maintain a solution equilibrium between the solid binary material and the ternary or quaternary melt. The ternary or quaternary melt is directionally cooled to grow the ternary or quaternary alloy. As a result, the ternary or quaternary melt becomes depleted in the two elements of the solid binary material, and a portion of the solid binary material dissolves into the ternary or quaternary melt to replenish these depleted two elements.
The present invention also relates to a homogeneous quaternary single-crystal alloy having the formula AxB1xe2x88x92xCyD1xe2x88x92y, x and y being the same or different and in the range of 0.001 to 0.999. The alloy is substantially free from crystal defects.